Things to Know When Using High-Purity Acetylene

      High-purity acetylene is a flammable gas with a colorless, aromatic odor. Acetylene produced from calcium carbide is toxic because it contains hydrogen sulfide (H₂S) and phosphine (PH₃), giving it a distinctive smell. Its melting point at 118.656 kPa is -80.8°C, and its boiling point is -84°C. Its relative density is 0.6208 (-82°C/4°C), and its refractive index is 1.00051 at 0°C and 1.0005 at 0°C. The flash point is -17.78°C, and the autoignition temperature is 305°C. In air, its explosive limits range from 2.3% to 72.3% by volume. It poses a severe explosion hazard both in liquid and solid states as well as in gaseous form under certain pressures. Factors such as heat, vibration, and electric sparks can trigger explosions; therefore, after being pressurized and liquefied, acetylene cannot be stored or transported safely. It is slightly soluble in water but readily dissolves in ethanol and benzene. At 15°C and 1.5 MPa, the solubility of acetylene reaches 237 grams per liter, and the solution remains stable under these conditions.

　　Therefore, in industrial applications, porous materials such as asbestos-filled cylinders are used to absorb acetylene gas. High-purity acetylene is then compressed into these cylinders for storage and transportation. To distinguish it from other gases, acetylene cylinders are typically painted milky white, rubber gas hoses are usually black, and the threads on acetylene hoses generally feature a threaded design (with radial intermittent grooves on the nut).

　　The reason why high-purity acetylene exhibits weak acidity is that the carbon-hydrogen bond in the acetylene molecule is formed by sp-s orbital overlap. The carbon atoms in both carbon and hydrogen exert a stronger attraction on electrons, resulting in a significantly higher electron cloud density near the carbon atom along the carbon-hydrogen bond. This makes the carbon-hydrogen bond polar, enabling it to donate a proton (H⁺) and thus exhibiting a certain degree of acidity (pKa = 25). When acetylene is introduced into an ammonia solution containing cuprous chloride, it immediately forms white silver acetylide (AgCCAg) and brownish-red cuprous acetylide (CuCCCu) precipitates, making this reaction useful for the qualitative identification of acetylene. When these two metal acetylides are dried, they become highly sensitive to heat or impact and can easily explode. For instance, after the reaction, they should be decomposed using hydrochloric acid or nitric acid to avoid potential hazards. Note: During use, storage, and transportation, acetylene should be kept away from contact with copper.

　　When burned at high purity, acetylene produces extremely high temperatures—its oxyacetylene flame can reach around 3,200 degrees Celsius—and is widely used for cutting and welding metals. With an adequate supply of air, acetylene burns completely, emitting a bright white light. In areas where electric lighting is not widespread or unavailable, acetylene can serve as a source of illumination. Acetylene is chemically reactive and can undergo reactions with numerous reagents. Before the 1960s, acetylene was an important raw material in organic synthesis, and it remains one of the key feedstocks to this day. For example, acetylene can be added to acetic acid to produce raw materials for polymer manufacturing.

　　Under different conditions, high-purity acetylene can undergo various polymerization reactions, yielding either vinylacetylene or divinylacetylene. At temperatures between 400 and 500°C, acetylene can undergo a cyclic trimerization reaction to form benzene. When using nickel cyanide Ni(CN)₂ as a catalyst, acetylene can be converted into cyclooctatetraene at 50°C and pressures ranging from 1.2 to 2 MPa.